Plastic high and low temperature tensile force testing is a critical evaluation method used to determine how plastic materials perform under tensile loading across extreme temperature conditions. This comprehensive guide covers the testing principles, applicable standards such as ISO 527 and ASTM D638, specimen requirements, test procedures, and typical industry applications. Whether you are developing automotive components, electronic enclosures, or aerospace parts, understanding temperature-dependent tensile behavior is essential for material selection and product reliability.
Technical Overview
High and low temperature tensile force testing of plastics measures the tensile properties of polymeric materials when the test is conducted inside an environmental chamber that simulates extreme thermal conditions. Plastics are inherently sensitive to temperature: as temperature increases, molecular mobility rises, causing a reduction in tensile strength and elastic modulus along with an increase in ductility. Conversely, at low temperatures, most plastics become stiffer and stronger but more brittle, with significantly reduced elongation at break.
The test involves clamping a standardized specimen into a universal testing machine equipped with a temperature-controlled chamber, applying a uniaxial tensile load at a constant crosshead speed until failure, and recording the force-extension curve. From this curve, key parameters such as maximum tensile force, tensile strength, yield stress, tensile stress at break, elongation at break, and tensile modulus are calculated at the specified temperature point.
The principal international and national standards governing this testing include ISO 527-1 and ISO 527-2, ASTM D638, GB/T 1040.1 and GB/T 1040.2, and JIS K 7161 and JIS K 7162. These standards define specimen geometry, gauge length, crosshead speed, and data calculation methods, ensuring reproducible and comparable results across accredited third-party laboratories worldwide.
Testing Samples
Dumbbell-Shaped (Dog-Bone) Specimens
The most common specimen geometry is the dumbbell or dog-bone shape, such as ISO 527-2 Type 1A (overall length 170 mm, gauge length 50 mm) or Type 1B, and ASTM D638 Type I specimens. These shapes concentrate deformation in the narrow central section, promoting failure within the gauge length. Specimens are typically produced by injection molding, compression molding, or machining from sheets and finished parts.
Rectangular Strip Specimens
For films and thin sheets, ISO 527-3 specifies rectangular strip specimens, typically 10 mm to 25 mm in width, tested with appropriate grips and gauge lengths. Flexible films may require extended gauge lengths to capture meaningful elongation data.
Specimens Cut from Finished Products
When evaluating the actual performance of a manufactured component, small-scale dumbbell specimens or strips can be die-cut or machined from finished products, such as cable jackets, pipe segments, and injection-molded parts. The specimen orientation relative to processing flow direction must be recorded, as anisotropy can significantly affect results.
Specimen Conditioning
Prior to testing, specimens must be conditioned according to ISO 291 or ASTM D618, typically at 23 °C and 50% relative humidity for at least 40 hours, unless otherwise specified. For high or low temperature tests, specimens must additionally be soaked in the environmental chamber at the target temperature for a sufficient equilibration period, commonly 10 to 30 minutes depending on specimen thickness, before loading begins.
Testing Items
The following tensile parameters are typically determined at each specified temperature point:
- Maximum tensile force (N or kN)
- Tensile strength (MPa), the maximum stress sustained by the specimen
- Tensile stress at yield (MPa)
- Tensile stress at break (MPa)
- Tensile strain and elongation at yield (%)
- Elongation at break (%)
- Tensile modulus (MPa), measured as the secant modulus between 0.05% and 0.25% strain per ISO 527-1
- Force-extension or stress-strance curve characteristics at each temperature
Testing Methods
The general test procedure is outlined below, based on ISO 527 and ASTM D638 combined with environmental chamber conditioning:
- Specimen preparation: Verify specimen dimensions with a calibrated micrometer or caliper, measuring thickness and width at multiple points within the gauge length and computing the cross-sectional area from the minimum values.
- Chamber conditioning: Place the specimen in the environmental chamber mounted on a universal testing machine. Common temperature points include -40 °C, -30 °C, 0 °C, 23 °C (room temperature reference), 70 °C, 85 °C, 100 °C, 125 °C, 150 °C, and higher for engineering and high-temperature polymers. Allow the specimen to equilibrate at the set temperature.
- Gripping and gauge length setting: Clamp the specimen using pneumatic, hydraulic, or wedge-action grips suitable for the temperature range, and set the extensometer or video strain measurement system to the specified gauge length.
- Loading: Apply the tensile load at the constant crosshead speed specified by the standard, typically 1 mm/min for modulus determination and 5 mm/min, 50 mm/min, or 200 mm/min for strength and elongation measurement depending on specimen type.
- Data acquisition: Record force and extension continuously until specimen failure, capturing the complete stress-strain curve.
- Calculation and reporting: Calculate tensile stress, strain, modulus, and strength values; validate failure location (failures outside the gauge length are typically retested); and report results together with temperature, humidity, specimen details, and test speed.
For statistically valid results, a minimum of five valid specimens per temperature condition is recommended. Temperature calibration of the chamber should be verified with traceable sensors, and the temperature uniformity within the working volume should comply with the tolerance requirements stated in the relevant standard.
Applications
Temperature-dependent tensile testing supports material qualification and design across many industries:
- Automotive: Under-hood plastics, interior trim, battery housings, and connector systems must retain tensile performance from -40 °C cold-start conditions to engine-bay temperatures exceeding 120 °C.
- Electronics and electrical: Enclosures, cable insulation, and connector plastics are evaluated for tensile retention at both low ambient temperatures and elevated operating temperatures.
- Aerospace: Cabin and structural polymer components require verified tensile properties across wide temperature envelopes, including cold soak at high altitude and heat exposure near engines.
- Packaging: Films and flexible packaging materials are tested to ensure sealing strength and puncture resistance are maintained during frozen storage or hot-filling processes.
- Construction and pipelines: Pressure pipes, geomembranes, and profiles are characterized at service temperatures to validate long-term mechanical reliability.
- New energy: Lithium battery separators, module frames, and photovoltaic encapsulants are tested at both low and high temperatures to confirm safety margins.
- Quality control and R&D: Comparative testing across temperatures helps formulators optimize resin grades, filler content, and additives for balanced stiffness, strength, and ductility.
FAQ
What temperature range can be tested?
Standard environmental chambers attached to universal testing machines typically cover -70 °C to +300 °C, with liquid nitrogen-assisted systems reaching as low as -180 °C for specialized applications. Common test points for plastics range from -40 °C to +150 °C.
Which standards are used for plastic tensile testing at temperature?
The core standards are ISO 527-1 and ISO 527-2, ASTM D638, and the corresponding Chinese standards GB/T 1040.1 and GB/T 1040.2. JIS K 7161 and JIS K 7162 are the Japanese standards harmonized with ISO 527. Conditioning atmospheres are defined in ISO 291 and ASTM D618.
How long must the specimen soak at the target temperature before testing?
Typical practice requires 10 to 30 minutes of thermal equilibration at the set temperature, depending on specimen thickness and thermal mass. Some specifications call for 1 hour or longer for thick or low-conductivity materials to ensure the entire cross-section reaches the target temperature.
Why does tensile strength decrease at high temperature?
Elevated temperature increases polymer chain segment mobility and reduces intermolecular forces, lowering modulus, yield stress, and ultimate strength while increasing ductility. Above the glass transition temperature, amorphous plastics show dramatic softening, whereas semi-crystalline plastics retain some load-bearing capability until approaching their melting point.
What crosshead speed should be used?
The speed depends on the standard and specimen type. Under ISO 527-2, Type 1A specimens are typically tested at 1 mm/min for modulus and 50 mm/min for strength; under ASTM D638, Type I rigid plastics are commonly tested at 5 mm/min. The selected speed must be stated in the test report because plastics are strain-rate sensitive, especially at high temperatures.
How many specimens are required per condition?
A minimum of five valid specimens per temperature condition is recommended by both ISO 527 and ASTM D638. Specimens that fail outside the gauge length or at grips are usually discarded and retested, as they underestimate the true material strength.
Can finished products be tested instead of molded specimens?
Yes. Specimens can be machined or die-cut from finished products when the intent is to verify actual component performance. The sampling location, orientation, and any deviation from standard geometry must be documented in the report.